1. Technical Field
The present disclosure relates to an optical/electrical hybrid substrate and its manufacturing method. More particularly, the disclosure relates to an optical/electrical hybrid substrate which includes a wiring substrate, an optical waveguide disposed on the wiring substrate, and a mirror for reflecting an optical signal, as well as to its manufacturing method.
2. Related Art
In recent years, with the increase in the speed of the information communication, light has come to be used as an information communication medium in place of an electrical signal. In such an optical communication field, it is necessary to convert an optical signal into an electrical signal and vice versa and to perform various kinds of processing such as modulation on light. To meet this requirement, optical/electrical hybrid substrates for such conversion processing are being developed.
FIG. 1 is a sectional view of the related-art optical/electrical hybrid substrate.
Referring to FIG. 1, the related-art optical/electrical hybrid substrate 200 includes a wiring substrate 201, an optical waveguide 202, mirrors 203 and 204, cladding members 206, an adhesive 207, a light-emitting element 208, a light-receiving element 209, and underfill resins 211 and 212.
The wiring substrate 201 includes a core substrate 215, through vias 216, upper traces 218, solder resist layers 219 and 223, solders 221, and lower traces 222.
The through vias 216 are provided to penetrate through the core substrate 215. The upper traces 218 are formed on a top surface 215A of the core substrate 215 and a top surface of the through vias 216. The top traces 218 have connection surfaces 218A to which a terminal 236 of the light-emitting element 208 and a terminal 238 of the light-receiving element 209 are connected. The solder resist layer 219 is formed on the top surface 215A of the core substrate 215 so as to cover parts of the top traces 218. The solder resist layer 219 has openings 219A through which the connection surfaces 218A are exposed when the openings 219A are not filled up. The solders 221 are formed in the openings 219A. The solders 221 serve to fix the terminal 236 of the light emitting element 208 and the terminal 238 of the light-receiving element 209 to the respective top traces 218.
The lower traces 222 are formed on a bottom surface 215B of the core substrate 215 and the bottom surface of the through vias 216. The lower traces 222 are electrically connected to the top traces 218 via the through vias 216, respectively. The lower traces 222 have connection surfaces 222A on which external connection terminals (not shown) are formed. The solder resist layer 223 is formed on the bottom surface 215B of the core substrate 215 so as to cover parts of the bottom traces 222. The solder resist layer 223 has openings 223A through which the connection surfaces 222A are exposed.
The optical waveguide 202 has a first cladding layer 226, a core 227, and a second cladding layer 228. The core 227, which serves to transmit an optical signal, is formed on the first cladding layer 226. The core 227 is made of a material having a larger refractive index than the first cladding layer 226 and the second cladding layer 228. The second cladding layer 228 is formed over the first cladding layer 226 so as to cover the core 227. The optical waveguide 202 has grooves 231 and 232 through which the core 227 is exposed when the grooves 231 and 232 are not filled up. The groove 231 is a V-shaped groove and has a inclined surface 231A on which the mirror 203 is formed. The inclined surface 231A is inclined so as to form a certain angle (45°, for example) with the top surface of the solder resist layer 219. The groove 232 is a V-shaped groove and has a inclined surface 232A on which the mirror 204 is formed. The inclined surface 232A is inclined so as to form a certain angle (45°, for example) with the top surface of the solder resist layer 219. The cladding members 206 fill the grooves 231 and 232 in which the mirrors 203 and 204 are formed, respectively.
The optical waveguide 202 in which the mirrors 203 and 204 and the cladding members 206 are formed is bonded to the top surface of the solder resist layer 219 with the adhesive 207.
The light-emitting element 208 has a light-emitting portion 235 and the terminal 236. The light-emitting element 208 is disposed on the wiring substrate 201 in such a manner that the light-emitting portion 235 is opposed to that part of the mirror 203 which is located in the core 227. The terminal 236 is formed on the connection surface 218A of the left-hand top trace 218. The terminal 236 is fixed to the left-hand top trace 218 with the solder 221.
The light-receiving element 209 has a light-receiving portion 237 and the terminal 238. The light-receiving element 209 is disposed on the wiring substrate 201 in such a manner that the light-receiving portion 237 is opposed to that part of the mirror 204 which is located in the core 227. The terminal 238 is formed on the connection surface 218A of the right-hand top trace 218. The terminal 238 is fixed to the right-hand top trace 218 with the solder 221.
The underfill resin 211 is formed so as to fill the space between the light-emitting element 208 and each of the wiring substrate 201 and the optical waveguide 202. The underfill resin 212 is formed so as to fill the space between the light-receiving element 209 and each of the wiring substrate 201 and the optical waveguide 202. The underfill resins 211 and 212 are made of an optically transparent resin.
FIGS. 2-8 show a manufacturing process of the above-described related-art optical/electrical hybrid substrate 200.
The manufacturing method of the related-art optical/electrical hybrid substrate 200 will be described below with reference to FIGS. 2-8. First, in a step of FIG. 2, a wiring substrate 201 is formed by a known method. Then, in a step of FIG. 3, a core 227 and a second cladding layer 228 are placed on a first cladding layer 226 in this order.
Then, in a step of FIG. 4, a groove 231 having a inclined surface 231A and a groove 232 having a inclined surface 232A are formed by processing the structure of FIG. 3 with a dicer. Then, in a step of FIG. 5, mirrors 203 and 204 are formed by forming metal films on the inclined surfaces 231A and 232A, respectively.
Then, in a step of FIG. 6, the groove 231 formed with the mirror 203 and the groove 232 formed with the mirror 204 are filled with respective cladding members 206. Then, in a step of FIG. 7, the optical waveguide 202 (the structure of FIG. 6) which is formed with the mirrors 203 and 204 and the cladding members 206 is bonded to the top surface of the solder resist layer 219 of the wiring substrate 201 shown in FIG. 2.
Subsequently, in a step of FIG. 8, the solders 221 are melted, a terminal 236 of a light-emitting element 208 and a terminal 238 of a light-receiving element 209 are fixed to the connection surfaces 218A of the upper traces 218, respectively, and underfill resins 211 and 212 are formed. The optical/electrical hybrid substrate 200 is thus completed (see e.g., JP-A-2000-304953).
As described above, in the related-art optical/electrical hybrid substrate 200, the grooves 231 and 232 in which the mirrors 203 and 204 are to be formed are formed in the optical waveguide 202 and then the mirrors 203 and 204 are formed on the inclined surfaces 231A and 232A of the grooves 231 and 232.
However, there is a problem in that it is difficult to form, with high accuracy, the inclined surfaces 231A and 232A at the certain angle (45°, for example) in the optical waveguide 202 in which the core 227 and the second cladding layer 228 are formed on the first cladding layer 226 in this order. This results in a problem that when an optical signal is transmitted via the mirrors 203 and 204 which are formed on the inclined surfaces 231A and 232A of the grooves 231 and 232, the optical signal suffers a large transmission loss.